Fused silica ceramic bodies are practical and advantageous for diverse applications. They can be easily and relatively inexpensively manufactured through slip casting and other methods. They feature a number of beneficial properties such as strength, durability and light weight. In certain special applications, fused silica ceramics display additional beneficial properties. One such special application is as an electromagnetic window, such as a “radome” covering for radar antennas and electronics, and fused silica radomes can often be found in aerospace applications such as aircraft or missile nosecones. The dielectric properties of fused silica make it especially suitable for a radome application, as fused silica does not significantly interfere with the radio frequency energy being transmitted and received by the antenna. Another special application making use of fused silica's advantages is pharmaceutical mixing; ceramic pestles and crucibles are often used. In this application, again, the silica is strong and lightweight, adjusts well to changes in temperature, and resist crumbling. The silica does not interact with most reagents being mixed, and ceramic utensils resist staining, do not rust or oxidize, and are relatively easy to clean.
However, fused silica bodies also have certain drawbacks in these various applications. Significant among these drawbacks is that fused silica, unless fully dense, is porous and allows moisture and other fluids to penetrate the wall thickness from one side of the body to the other. This may be unacceptable for several reasons, the specific reasons depending on the particular application. For example, in the case of slip cast fused silica bodies used as radomes in external or aerospace applications, the semi-porous fused silica wall permits moisture from the outside atmosphere to penetrate the radome wall and potentially reach the sensitive electrical components located inside that are critical to system performance. These sensitive components benefit from environmental protection over extended periods of time. In these applications, seepage of moisture presents a serious difficulty for radar equipment operational quality, or even survival in inhospitable environments. As another example, in the pharmaceutical mixing application the semi-porous fused silica permits external moisture to penetrate the vessel wall and contaminate the chemicals that are being compounded inside the vessel. The rough texture of the silica also permits reagent residue to cling to the interior of the vessel and resist complete removal.
In aerospace applications where fused silica radomes have been used without any protective coating or barrier, the moisture penetration problem has made it necessary to further enclose the sensitive electronic equipment inside the radome within its own environmentally sealed enclosure. Where attempts have been made to seal the radome itself, use has been made of organic, inorganic, or combination organic-inorganic coatings as moisture transmission barrier layers. These material layers have typically been chosen to minimize interference with the dielectric performance of the radome and to match relevant material properties of the radome body. Some of these coatings have been used with fair success in conjunction with certain other types of radome materials but have had varying and uneven results with fused silica. In many cases, there is a significant capital investment to place these thin (usually less than ten micrometer) coatings onto a radome. In addition, many of these coatings can only be applied to a radome exterior, leaving the coating susceptible to damage from normal handling and wear and tear. In some cases application of the coating materials may require a heat treatment that can significantly affect the material properties of the base radome body and damage the moisture barrier coating itself. Further, the addition of these materials may degrade the RF dielectric profile and performance of the radome.